This invention relates to the polymerization of olefins, such as ethylene, in the presence of any polymerization catalyst and one or more novel chromium cocatalyst systems.
It is well known that olefins, such as ethylene, can be polymerized with catalyst systems employing supported, chromium catalyst or magnesium/titanium catalysts. Initially such catalysts were used primarily to form olefin homopolymers. It soon developed, however, that many applications required polymers which were more impact resistant than olefin homopolymers. Consequently, in order to produce polymer having short chain branching like the more flexible free radical polymerized olefin polymers, comonomers such as propylene, butene, hexene or other higher olefins were copolymerized with the olefin monomer to provide resins tailored to specific end uses. The copolymers, however, are more expensive to produce since inventories of different monomers must be kept and also the comonomers are generally more expensive than the usual short-chain monomers, such as ethylene or propylene. Linear olefin polymers, such as polyethylene, with short chain branching can be formed from a pure ethylene feed using the old free radical high pressure process, but the conditions necessary to do this make the product too expensive to be commercially competitive.
Additional control over the polymerization process and the resultant polymer is also desired. A process to consistently reduce the density of linear olefin polymers and to more efficiently produce and incorporate comonomers into the linear olefin polymer is economically advantageous. A shift in the polymer branch distribution, wherein the branch length is decreased and the amount of branching is increased, is also economically desirable.